Manganese dioxide has been proposed as a positive electrode active material for a sealed alkaline storage battery using zinc as a negative electrode active material (Japanese Patent Publication No. 45-3570). Also, a mixture of nickel oxide and manganese dioxide has been proposed as a positive electrode active material for an alkaline primary battery using zinc as a negative electrode active material (Japanese Laid-Open Patent Publication No. 49-114741).
However, manganese dioxide is poor in reversibility in a charge-discharge reaction and is difficult to return to manganese dioxide by charge after discharge. Therefore, the utilization of the active material is rapidly lowered through repeated charge-discharge cycles, resulting in rapidly decreasing the discharge capacity. Furthermore, the oxygen evolution potential of manganese dioxide is so low that the pressure within the battery is increased due to an oxygen gas generated through decomposition of water on the positive electrode during charge. As a result, the adhesion in a connecting portion of a battery housing member is degraded, so that the electrolyte can easily leak.
On the other hand, when the mixture of nickel oxide and manganese dioxide is used in a storage battery (secondary battery) that is repeatedly charged and discharged, the oxygen evolution potential of the mixture is so low that the pressure within the battery can be easily increased during charge and the electrolyte can easily leak similarly to the battery using manganese dioxide. Furthermore, manganese dioxide included in the mixture is poor in reversibility in a charge-discharge reaction, and hence, the utilization of the active material is rapidly lowered through repeated charge-discharge cycles, resulting in rapidly decreasing the discharge capacity.
In this manner, both of the positive electrode active materials are too disadvantageous to be used as a positive electrode active material for a sealed alkaline storage battery. The increase of the pressure within a battery during charge and the resultant leakage of the electrolyte are particularly significant in a sealed alkaline storage battery including an active material in a large amount.
Accordingly, an object of the invention is providing a highly reliable sealed alkaline storage battery including an active material in a large amount but hardly suffering electrolyte leakage for a long period of charge-discharge cycles.
Another object of the invention is providing a sealed alkaline storage battery that can keep high utilization of an active material not only in initial stages of charge-discharge cycles but also for a long period of time.